The field of the present invention relates generally to a process for recovering gold or other precious metals from refractory ores by pressure oxidation followed by carbon-in-leach treatment.
Gold and other precious metals naturally occur in ores in several different forms and complexes. Unfortunately, however, the gold-bearing ores found in Nevada, Utah, California and other states in the United States, as well as other countries throughout the world, often contain refractory material which interferes with the extraction of gold and other precious metals. Further, the actual gold content of such ores is variable and an ore with a relatively small actual gold content can have less than one-tenth of an ounce of gold per ton of mined ore. When an ore with a relatively small actual gold content is processed, the adverse effects of refractory material may make the recovery of gold prohibitive unless an effective process is utilized for coping with the refractory material.
In the prior art, many processes are disclosed for treating "sedimentary carbonaceous gold-bearing ores" or refractory ores. However, these processes rarely define the actual constituents posing the problem to the particular process for a particular ore. Thus, as noted by W. J. Guay and M. A. Gross in Preprint 81-34 of the Society of Mining Engineers of AIME, the term "carbonaceous" has been rather loosely applied to ore constituents of widely varying characteristics, including: (1) an activated carbon component capable of adsorbing gold chloride or gold chloride complexes from solutions; (2) a mixture of high molecular weight hydrocarbons usually associated with the activated carbon components; and (3) an organic acid, similar to "humic acid", containing functional groups capable of interacting with gold complexes to form organic gold compounds. While the structure of such compounds is unknown, it is possible that they are formed by chelation wherein ligands, such as N, S or O, in organic acids form stable gold chelates.
In addition to the arbitrary nature of any definition of the term "carbonaceous", the term "refractory" has also been the subject of a rather unsettling, all-inclusive, vague definition. Thus, loosely speaking, the term "refractory" has been used to define an ore containing any substance which interferes with the recovery of gold from said ore by standard cyanidation techniques. One such substance which is commonly known to render an ore refractory is pyrite, which may occlude finely disseminated particles of gold in spheroidal or cubic clusters. Such occluded gold particles may have a size of less than .02 microns. Other materials which may be deemed to render an ore refractory, besides the general class of carbonaceous materials, include clay minerals which can adsorb the gold cyanide complex and certain sulfur bearing compounds other than pyrite.
In the past, extensive research has been conducted into methods for dealing with the problem of carbonaceous impurities in gold-containing ores. Some of these studies have indicated that the carbonaceous material comprises active carbon which appears to adsorb the gold cyanide complex Au(CN).sub.2.sup.- from cyanide leaching solutions, as well as long chain organic compounds which appear to form stable complexes with the gold. In an attempt to overcome some of these problems, the United States Bureau of Mines has conducted experiments in which they used a wide variety of oxidation pretreatment systems including ozone, sodium hypochlorite, calcium hypochlorite, permanganates, perchlorates, chlorates and oxygen. As a result of these and other experiments, various processes dealing with pretreatment systems have been patented, including U.S. Pat. No. 1,461,807, which discloses the use of certain mineral oils for "blinding" the action of the carbonaceous impurities on the cyanide complex formation; U.S. Pat. No. 2,234,140, which discloses that certain wetting agents can make the ore more amenable to cyanidation; U.S. Pat. No. 3,639,925, which discloses the use of sodium hypochlorite and calcium hypochlorite as agents for oxidizing the carbonaceous materials so as to prevent them from adsorbing the gold cyanide; U.S. Pat. No. 3,846,124, which discloses a chlorine pretreatment of the ore in the absence of any alkaline material in order to decompose the organic carbonaceous components and remove them prior to cyanidation; U.S. Pat. No. 3,574,600, which discloses that certain acids can be used in conjunction with an ozone treatment prior to cyanidation to oxidize the carbonaceous impurities; and U.S. Pat. No. 4,038,362, which discloses a pre-oxidation technique carried out in the absence of extraneous alkaline material for reducing the amount of chlorine needed to pretreat the ore.
Another process utilizing air oxidation followed by chlorination prior to cyanidation is disclosed by W. J. Guay in the article "How Carlin Treats Gold Ores by Double Oxidation" in World Mining, March, 1980, pages 47-49. In this process, an ore bearing gold of which a substantial portion of the ore was not amenable to standard cyanidation techniques because of the presence of activated carbon in pyrite is disbursed in an aqueous slurry of ground ore at 40-50% solids and temperatures of 80.degree.-86.degree. C. until a considerable portion of the pyrite is oxidized to iron oxides. Although some of the carbonaceous materials are decomposed by the oxidation, it was found necessary to follow the air oxidation with chlorination in order to complete the oxidation of the carbonaceous materials and pyrite. This process also utilized the addition of a solution of sodium carbonate over a period of several hours during the air oxygenation to react the air and the soda ash with pyrite to form soluble sulfates and iron oxides. The soda ash is added to the slurry in amounts varying from 25- 100 pounds per ton of ore.
In addition to various processes for the pretreatment of carbonaceous ore, other processes have been disclosed which involve modifications to the actual cyanidation process. Thus, U.S. Pat. Nos. 2,147,009 and 2,315,187 disclose the use of finely divided charcoal during cyanidation to simultaneously leach the gold from the ore and absorb the gold on the charcoal so as to maintain the solution continuously depleted of gold and thereby improve cyanidation efficiency. More recently, the prior processes utilizing finely divided charcoal have been improved by replacing the finely divided charcoal with granular activated charcoal. Such a process, which is commonly known in the art as "carbon-in-leach," is disclosed in U.S. Pat. No. 4,289,532, the disclosure of which is hereby specifically incorporated by reference, in which a slurry is initially treated with an oxygen-containing gas for at least an hour and contacted with the source of hypochlorite ions for at least one hour, followed by the simultaneous contact of the oxidized aqueous slurry, in a plurality of stages, with a cyanide complexing agent and granular activated carbon, the carbon flowing in countercurrent fashion with the slurry such that the gold is transferred to the granular activated charcoal. However, this process still consumes large amounts of cyanide, which may be the single most expensive operating cost in an ongoing gold extraction process, and the process is heavily time-consuming because of the retention times involved in the oxidation and chlorination steps.